The overall objective of this proposal is to obtain a better understanding of the molecular aspects of Schwann cell myelination. Specific Aim 1 will characterize the role that microtubules play in the transport of myelin protein. These studies will extend our previous work that has described the subcellular sites of myelin protein synthesis and the ultrastructural localization of myelin protein. We will determine whether microtubule depolarization (produced by colchicine) alters the morphology of myelinating Schwann cells. The immunocytochemical distribution of myelin proteins will also be determined in tissue sections of colchicine-treated nerves. If microtubules are involved in transport, we expect myelin proteins to accumulate in Schwann cell perinuclear cytoplasm. Other studies will investigate basic features of Schwann cell microtubules, such as their orientation (polarity) and their 3-dimensional relationship with the Golgi apparatus and microtubule organizing center. The myelin-associated glycoprotein (MAG) is a minor constituent of PNS myelin and a member of the immunoglobulin gene superfamily. We and others have postulated that MAG functions as an adhesion molecule in membrane-membrane interactions that are essential to normal myelination. To address this possibility more directly, studies in the second specific aim of this proposal are designed to produce a MAG-deficient mouse by introducing MAG anti-sense DNA into Schwann cells of transgenic mice. Dramatic reduction of MAG in myelinating Schwann cells may provide compelling evidence for the function of MAG during axonal interaction and spiral growth of the myelin sheath. The third specific aim of this proposal is to begin to characterize the role that receptor-mediated endocytosis plays during active myelination. Electron microscopic studies will determine where this process occurs and whether it is developmentally regulated by determining the subcellular distribution of clathrin-coated pits. These studies will represent the first systematic investigation of receptor-mediated endocytosis in myelin Schwann cell and they should provide fundamental understanding of basic aspects of this process as it relates to myelinogenesis.